This invention relates to machine tools, and more particularly to tapping attachments for machine tools having a rotating workpiece spindle.
Generally, tapping is accomplished by first advancing a tapping tool into a hole-to-be-tapped with the tool rotating at a predetermined angular velocity in a first direction with respect to the workpiece, and then retracting the tool from the hole with the tool rotating at a predetermined angular velocity in the opposite direction. In practice, there have been many approaches to performing this operation. For example, with a conventional tapping tool having axial float and adapted for self-feeding, the tool may be held stationary while the workpiece is rotated in a first direction during advancement into the hole, and rotated in the opposite direction during retraction. Conventional reversible gearing may be used to control the workpiece spindle in this approach. Alternatively, the workpiece may be held stationary while the tapping tool is rotated in a first direction during the advancement portion of the cycle and in the opposite direction during the retraction portion of the cycle. The latter approach may be accomplished using reversible gearing configurations for coupling the tapping tool to the tool drive shaft. Alternatively, a constant velocity tool drive may be used in conjunction with a conventional reversible tapping attachment.
Under conditions when the appropriate speed drives and gearing configurations for the spindle and tapping tool drive shaft are available, the above-noted approaches are satisfactory. There are, however, many practical situations where these speeds are not available. For example, in a multiple spindle automatic screw machine, a plurality of workpiece spindles and associated coaxial tool positions are arranged in a multiple work station configuration so that a multiplicity of operations may be performed simultaneously on a machine having a single drive motor and associated main power shaft. Generally, the spindles and tool positions are disposed symmetrically about a central axis and the spindles are coupled on a carrouselled carrier which may be indexed about the central axis so that each spindle may be stepped around to each tool position. With this configuration, appropriate tools may be used as the various tool positions so that a succession of machining operations may be performed on all workpieces as the carrier is stepped about the central axis, remaining at each position for a period long enough to complete the longest operation in the succession. Since most commonly used machining operations (such as drilling) are best performed with the workpiece rotating at a relatively high speed with respect to the tool, multiple spindle machines are typically set-up to provide such a ratio of speeds for each work station. However, this ratio is generally an order of magnitude too high for tapping operations. Thus, for a multiple station machine where all the spindles are turning at speeds that are suitable for the majority of machining operations, tapping may be accomplished only with adaptive elements to reduce the speed ratio.
In the prior art, special gearing arrangements have been provided for adaptation to the machine's main power shaft using two friction clutches and a two-point pick-off. With this configuration, the workpiece spindle for a tapping work station is maintained at the normal speed while the gear and clutching arrangement adjusts the speed of the tool drive shaft so that the tapping tool may selectively be controlled to rotate at a speed somewhat greater than the workpiece speed, or at a speed somewhat slower than the workpiece speed. The gearing for coupling the tool drive shaft to the main power shaft is appropriately selected so that the differential speeds of the tapping tool with respect to the workpiece are appropriate for the desired tapping operation. However, this arrangement requires relatively complex and expensive gearing for controlling the tool drive shaft from the main power shaft. Furthermore, substantial set-up times are required for the installation of such gearing in conventional multiple spindles screw machines, thereby placing severe limits on machine productivity.
Accordingly, an object of the present invention is to provide a tapping attachment for a machine having a rotating workpiece spindle and tool drive shaft, where the tapping attachment couples a tapping tool to the drive shaft.